This set of initials is one of the most fluid now in circulation. Originally, it meant a type of health maintenance organization allowing physicians to work out of their own offices instead of a central facility. These HMOs typically are formed and operated by physicians and marketed to employers. Under this arrangement, physicians still see their individual patients, as well as patients from the HMO. Today, this set of initials also includes entities that contract with groups of providers, including corporate providers such as behavioral health centers. Each provider agrees to see patients/clients from plans who contract with the HMO of which the IPA is a part and to serve these clients for agreed-upon fees. Such IPAs often are not associated with a single HMO, but serve a number of them under contract. This is especially the case in specialty services such as behavioral health. This model is another example of entities that are in the "wholesale" as opposed to the retail business, since IPAs tend to contract with payers who market their own plans to employers